This invention relates to a vehicle air conditioning or climate control system, and more particularly to a method and apparatus for biasing the operating point of the system as required to prevent the build-up of odor producing microorganisms.
The production of offensive odors in vehicle air conditioning systems has been traced to the build-up of certain types of microorganisms on the surface of a wet evaporator core. The odor problem can occur in any air conditioning system but is most prevalent in energy efficient systems that operate the evaporator at higher than traditional temperatures in order to minimize series re-heating of evaporator outlet air to achieve a desired air discharge temperature. These issues have been generally recognized in the motor vehicle industry, as demonstrated for example, in the U.S. Pat. No. 6,035,649 to Straub et al. issued on Mar. 14, 2000. Specifically, Straub et al. posit that the odors are caused by frequent changing of the evaporator state between wet and dry as the surface temperature of the evaporator oscillates about the dew point temperature of the intake air, and therefore teach that the surface temperature of the evaporator must be continuously maintained either above or below the dew point temperature by determining the dew point temperature and controlling the evaporator temperature accordingly. However, only limited dehumidification can be achieved when the evaporator is maintained above the inlet air dew point temperature, and adequate air conditioning performance in many situations requires the evaporator surface temperature to be maintained below the inlet air dew point temperature. Indeed, we have found that maintaining the evaporator surface temperature continuously below the inlet air dew point temperature virtually ensures odor-free operation because the condensate continuously cleanses the evaporator surface of odor causing microorganisms.
While a control of the type described by Straub et al. can be used to effectively prevent air conditioning odors by maintaining the evaporator surface temperature below the inlet air dew point temperature, it requires the expense of a dew point sensor or a relative humidity sensor in order to determine the inlet air dew point temperature. Since such sensors add considerable cost to an air conditioning system, what is needed is a control that uses only inexpensive sensors to maintain the evaporator at an odor-free operating point.
The present invention is directed to an improved air conditioning method and apparatus including an evaporator that is chilled by refrigerant, where the presence of sufficient condensate flow for odor-free operation is detected based on the surface temperature of a thermistor or other electrically activated temperature sensor disposed in a condensate drainpipe of the evaporator.
In a first embodiment, the surface temperature of the drainpipe sensor is used to calculate the temperature of a stagnant fluid (air or water) in the drainpipe based on the power supplied to the sensor and the convective heat transfer characteristics of air and water. If the calculated temperature of stagnant air is approximately equal to the evaporator temperature, it is deduced that there is little or no condensate flow through the drainpipe; in this case, the evaporator is too dry and the operating point of the air conditioning system is lowered to reduce the surface temperature of the evaporator. If the calculated temperature of stagnant water is approximately equal to the evaporator temperature, it is deduced that the drainpipe is plugged; in this case, the refrigerant compressor is disabled and the operator is advised to have the air conditioning system serviced. Otherwise, the evaporator is deemed to be generating sufficient condensate to cleanse the evaporator surface of odor causing microorganisms, and there is no adjustment of the operating point of the air conditioning system.
In a second embodiment, a constant power is supplied to the drainpipe sensor, and the state of the evaporator is deduced by comparing the surface temperature of the sensor to a set of predefined reference temperatures. The predefined reference temperatures are experimentally determined for different operating conditions of the evaporator, including at least a condition for which the evaporator is too dry, and a condition for which the evaporator drainpipe is plugged. If it is deduced that the evaporator is too dry, the operating point of the air conditioning system is lowered to reduce the surface temperature of the evaporator. If it is deduced that the drainpipe is plugged, the refrigerant compressor is disabled and the operator is advised to have the air conditioning system serviced.